Synthetic models of electron-transfer proteins are being studied with a variety of physical techniques. The fundamental nature of thermal electron transfer between transition metal ions is being elucidated through study of binuclear mixed-valence iron complexes. A number of spin-crossover ferric complexes have been prepared with the goal of characterizing: 1) the spin-flipping rate for an individual complex; and 2) the nature of the cooperativity of spin-flipping that is found in the solid state. Iron-quinone complexes, including those with porphyrin ligands, have been prepared and are being characterized with several physical techniques such as variable-temperature magnetic susceptibility, EPR, and 57Fe Mossbauer. Magnetic exchange interactions are being studied as they are found for binuclear metal complexes with extended bridging groups.